Preuni

The more I read all the threads about what speed differences between the same boats with the same engines and props can be, the more I wonder how differences of up to 5mph can be explained.
How does elevation influence the horsepower output of a modern boat engine?......I heard theres a 10mbar drop in pressure with every 300 ft of altitude.....this would mean that at 1200 ft the ambient pressure would be 40 mbars lower than on sea level...... Given that the pressure at mean sea level is about 1000mbar, that would mean a loss of 4% pressure and therefore 4% loss of horsepower (right??)....on a 425 hp engine thats almost 20 hp, and 20 hp on a 27 ft., 4500 pound boat should be good for a 1 or 2 mph difference!
Has anyone experienced that effect with one and the same boat at different locations or is it just theory?
How does saltwater influence topend speed on a boat in comparison to freshwater....the density of saltwater is alot higher than on freshwater, in theory boats should run higher and therefore more dry and with less drag...any real world experiences out there?
Just searching for an explanation why my 272 with an 496HO is so much slower than the exact same boat of a friend....He is running 66 GPS on salt and on sea level, mine runs only 62 GPS on fresh water at 1300 ft.....

FOUNTAINATLAST

I can't anwser for driving on salt water but can atest to different speeds at different altitudes. I also run a 496HO and normally boat at Lake Pleasant which is about 1700' above sealevel and run 65 5000 RPM, then travel to Lake Havasu which is only 400' above sealevel and run 69 5300. Don't know why but my best guess is the thinner air at higher elevation plays havoc with the MAP & BARO sensors???!!!

Reed Jensen

The boat hull has to displace less water because the water is denser with the salt. Also... since the water is denser... the prop has better "grip". Then there is the heavier air... hence... denser fuel air mixture. Am I making sense?... or do I sound dense?........